1. Field of the Invention
The present invention relates to polyepoxide compositions and more particularly to heat curable polyepoxide compositions suitable for encapsulating electrical components.
2. Description of the Prior Art
High voltage power supplies and pulse forming networks for aerospace use must meet high standards of performance and reliability for long periods under extreme environmental conditions. To assure trouble-free operation of components within the assembly, the components are commonly encapsulated or impregnated with synthetic resin materials to provide electrical insulation to the component and to protect the components and equipment from possible detrimental environmental effects which might cause failures of some sort. Thus, electrical components such as magnetic coils, capacitors and diode bridges are pre-encapsulated (i.e. coated with primary encapsulation) prior to assembly into the finished power supply. Primary encapsulation of components provides many advantages including excellent dielectric characteristics, mechanical ruggedness during assembly, ease of mounting and environmental resistance. However, in spite of these advantages the components are frequently found to be unreliable and fail during use due to the crevices or low pressure voids remaining in the insulation after the encapsulation process. These voids constitute areas of low dielectric strength. If these defects occur in high electric fields, the phenomenon known as corona or partial electrical discharge will occur, degrading the insulation and causing an eventual short circuit. To be acceptable for commercial use, the encapsulated electrical component, such as a magnetic coil, should encounter no corona at operating electrical stresses, e.g. 1000 volts per mil (V/mil).
Because of their excellent adhesion, good mechanical, humidity and chemical properties, epoxy resins are used extensively as encapsulating compositions for electrical and electronic components.
Current state-of-the-art, high reliability, electrical component primary encapsulants are generally composed of diglycidyl ethers of bisphenol A, epoxylated phenol-formaldehyde, novolac resins or combinations thereof. The epoxy resins are rendered heat curable by the incorporation of a stoichiometric amount of a curing or hardening agent such as an aromatic amine, for example, metaphenylene diamine or benzyldimethylamine. Athough these epoxy resin formulations have been used successfully as electrical and electronic encapsulating agents, such materials are rigid and not crack-resistant. Further, the viscosity of these materials, is still too high, e.g. 500 centipoise (cps) at 75.degree. F. (24.degree. C.), to be considered easily workable; and during encapsulation, these compositions are not intruded completely into voids and crevices present in the electrical components, which causes incomplete insulation. In addition to their high viscosities, these epoxy resin materials have long gel times and mold dwell (e.g. 45 minutes and 2 hours, respectively), as well as requiring vacuum processing (e.g. 20-30 micrometers mercury pressure) under vacuum/hydrostatic gas pressures. The long processing cycles and the use of high vacuum equipment are major factors in the high cost of encapsulated electrical products fabricated using these epoxy resins. There is thus a need in the art for higher reliability, faster curing epoxy resin compositions suitable as encapsulants which are capable of withstanding high electric stress.